Capsaicin (trans-8-methyl-N-vanillyl-6-nonenamide) is present in juices of Capsicum plants, and it has been commonly used as a spice, and further, it has been known to exhibit an analgesic effect and an antiinflammatory effect. Capsaicin has been considered to act on a specific receptor being present in primary afferent sensory nerves (C-fibers: capsaicin-sensitive nerves), whereby it causes a drastic pungency (pain) and further exhibits an analgesic effect and an antiinflammatory effect thereafter. Recently, said receptor has been cloned and named as vanilloid receptor subtype 1 (hereinafter, referred to as VR1) (cf., Nature, 389, 816 (1997)). VR1 has been considered from its amino acid sequence as an ion channel having six transmembrane domains and showing a high Ca2+ permeability, and it is suggested that VR1 is possibly activated not only by capsaicin-like compounds but also by thermal stimuli or protons, and further that VR1 may possibly be involved with pain in various clinical conditions. When capsaicin acts on VR1 in primary afferent sensory nerves, a cation channel is opened, and the membrane is depolarized so that neuropeptides such as substance P are released to induce pain. Capsaicin, a pain inducing substance, is actually used in the treatment of painful disorders such as diabetic neuropathy and rheumatic arthritis. The reason for this paradoxical use of capsaicin has been considered that sensory nerves are insensitive (desensitized) to painful stimuli as a result of continuous VR1 cation channel opening after repeatedly application of capsaicin (cf., Pharmacol. Rev., 51, 159 (1999)).
At the moment, narcotic analgesics (morphine, etc.) and nonnarcotic analgesics (NSAIDs, etc.) are commonly used as an analgesic agent. However, the use of narcotic analgesics is strictly restricted, because of development of tolerance, dependency or serious side effects thereof. Further, nonnarcotic analgesics are not effective to severe pain, and it has been known that nonnarcotic analgesics are associated with significant rates of upper digestive disorders or liver disorders by prolonged administration thereof. Under the circumstances, it has been desired to develop an analgesic agent exhibiting a higher analgesic effect with few side effects. Moreover, since there is no effective analgesic agent for neuropathic pains such as pains caused by diabetic neuropathy, post herpetic neuralgia, trigeminal neuralgia, it has also been desired to develop a novel analgesic agent to treat them.
Capsaicin-like compounds acting on VR1 receptor (VR1 receptor agonists) have been considered to exhibit their effects based on a completely different action mechanism from the existing analgesic agents, i.e., desensitization of capsaicin-sensitive sensory nerve, and hence, the efficacy of such compounds has greatly been expected for the treatment of neuropathic pains for which the existing analgesic agents cannot be effective, or pains induced in various pathologic conditions such as rheumatic arthritis.
Since it is reported that capsazepin, which is an only-known VR1 receptor antagonist, exhibits a significant analgesic effect in an animal pain model (cf., Life Science, 69, 2911 (2001)), VR1 receptor antagonists may also be an agent for treatment of various pains.
The compounds with similar actions of capsaicin may be useful as an agent for treatment of diseases in which primary afferent sensory nerves (C-fibers) are concerned, such as essential pruritus, allergy or nonallergic rhinitis, frequent urination and urinary incontinence with overactive bladder, stroke, irritable bowel syndrome, respiratory disorders such as asthma and chronic obstructive pulmonary disease, dermatitis, mucositis, gastric and duodenal ulcers, inflammatory bowel diseases.
Further, since it has been reported that capsaicin exhibits an anti-obesity activity by promoting the hypersecretion of adrenalin (cf., Pharmacol. Rev., 38, 179 (1986)), capsaicin-like compounds will be useful as an agent for treatment of obesity.
As capsaicin analogous compounds having an analgesic activity, JP-A-63-295537 (=EP 0282127B, U.S. Pat. No. 5,099,030, U.S. Pat. No. 5,045,565) discloses beta-aminoethyl-substituted phenyl compounds, JP-A-4-230257 (=EP 0462933B) discloses N-benzyl-N′-phenyl- and -phenylalkyl-thiourea compounds, and WO 00/16756 discloses N-acylvanillinamide derivatives. In addition, J. Med. Chem., 36, 2595 (1993) discloses with respect to the analgesic effects of N-(4-hydroxy-3-methoxybenzyl)-4-pentylbenzamide, but it is reported that this compound has no analgesic effect. The compounds disclosed in these patent publications and literatures are N-benzylamide derivatives or N-benzylthiourea derivatives, but the compounds of the formula (I) as described hereinafter are phenylacetamide derivatives, and therefore, these compounds are different in the chemical structures thereof.
It has been disclosed that the substituted phenylacetamide derivatives (in JP-A-62-48657 (=EP 206609B, U.S. Pat. No. 5,013,759)) and the N-arylalkylphenylacetamide compounds (in JP-A-5-320113 (=EP 0525360B, U.S. Pat. No. 5,242,944) and JP-A-8-283220 (=EP 0721939B, U.S. Pat. No. 5,670,546)) exhibit an analgesic effect. Furthermore, GB 2168975 discloses an aralkaneamide compound having an analgesic effect. However, these patent publications never specifically disclose the N-arylacetamide derivative of the formula (I) as described below.
WO 99/29674 discloses that the following anilides are compounds inhibiting the metabolism of retinoic acid.
wherein X is O, etc., R1 is hydrogen atom, a C1-6 alkyl group, etc., R2 is a hydrogen atom, a C1-12 alkyl group, a C2-8 alkenyl group, etc., R3 is a hydrogen atom, a C1-6 alkyl group, etc., R4 is an aryl group, etc., Het is an unsaturated heterocyclic group selected from a pyrrolyl group, a pyrazolyl group, an imidazolyl group, a triazolyl group, a tetrazolyl group and a pyrimidinyl group, which may optionally be substituted by an amino, a mercapto, a C1-6 alkyl, a C1-6 alkylthio, or an aryl.